Non-Kramers ESEEM of Integer-Spin Diferrous Carboxylate-Bridged Clusters in Proteins

The diferrous oxidation level of carboxylate-bridged non-heme diiron proteins is pivotal because only it reacts directly with dioxygen. In this state each ferrous ion has ST = 2. Such integer-spin, so-called “Non-Kramers” (NK) systems typically are difficult to access by paramagnetic resonance techniques, and this property has precluded the application of ENDOR and ESEEM spectroscopies to this state. Integer-spin systems frequently exhibit a ground state “Non-Kramers doublet”, however, which is split in zero applied field by an energy within the microwave range, thus rendering the center EPR active. Recently, we showed that ENDOR and ESEEM spectroscopies also can be applied to NK doublets, in preliminary measurements on diferrous methane monoxygenase hydroxylases (MMOHred) Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b and azido-hemerythrin (N3Hrred) (Hoffman, B. M.; Sturgeon, B. E.; Doan, P. E.; DeRose, V. J.; Liu, K. E.; Lippard, S. J. J. Am. Chem. Soc. 1994, 116, 6023−6024). Buildi...